This invention relates to electric power supplies, and particularly to a switching power supply capable of a.c. to d.c. voltage conversion, featuring provisions for attainment of closer approximation of the input current waveform to a sinusoidal wave, and a higher power factor.
A conversion from an alternating to a direct current is possible by a rectifying and smoothing circuit comprising a rectifying circuit having a diode connected to an a.c. power supply, and a smoothing capacitor connected to the rectifying circuit. This type of rectifying and smoothing circuit possesses the disadvantage, however, of a somewhat poor power factor as a result of the fact that the smoothing capacitor is charged only at or adjacent the peaks of the a.c. voltage of sinusoidal waveform. Another drawback is that it is incapable of adjustably varying the d.c. output voltage.
Japanese Unexamined Patent Publication No. 8-154379 represents an improvement of the rectifying and smoothing circuit above. It teaches a switching power supply comprising a rectifying circuit, a smoothing capacitor, a d.c.-to-d.c. converter circuit, and an inductive reactor for a higher power factor. The reactor is electrically connected between the pair of output terminals of the rectifying circuit upon closure of a switch included in the d.c.-to-d.c. converter circuit. The desired improvement in power factor is thus attained, as the current flowing through the reactor varies in amplitude in step with the a.c. input voltage.
Despite its undisputable advantages, this prior art switching power supply has proved to be not so satisfactory as can be desired in terms of power loss.
The present invention seeks to improve the switching power supply of the noted prior art type for still higher efficiency without impairment of its inherent advantages.
Briefly, the invention may be summarized as a switching power supply capable of translating a.c. voltage into d.c. voltage. Included is a transformer connected to a pair of a.c. input terminals via a rectifier circuit, and to a pair of d.c. output terminals via a rectifying and smoothing circuit. The rectifier circuit has a first and a second output, the second being grounded in the preferred embodiments disclosed herein. A smoothing capacitor is connected between a first extremity of the primary winding of the transformer and the second output of the rectifier circuit, and an inductor between the first output of the rectifier circuit and the smoothing capacitor via at least part of the transformer primary. A primary switch is connected between a second extremity of the transformer and the second output of the rectifier circuit. The primary switch is provided with soft-switching capacitance means which can take the form of either a discrete capacitor connected in parallel therewith or parasitic capacitance of its own.
The invention particularly features a soft-switching circuit which comprises an additional winding of the transformer, and an ancillary switch connected in parallel with the smoothing capacitor at least via the additional transformer winding. The ancillary switch is designed to supply to the additional transformer winding a current of sufficient magnitude to cause the transformer primary to develop a voltage that enables the soft-switching capacitance means to discharge. A switch control circuit is connected both to the primary switch for on-off control of the primary switch at a repetition frequency higher than the frequency of the a.c. input voltage, and to the ancillary switch in order to initiate conduction through the ancillary switch earlier than the beginning of each conducting period of the primary switch and to terminate conduction through the ancillary switch not later than the end of each conducting period of the primary switch.
Such being the improved construction of the switching power supply according to the invention, a current will flow through the inductor during the conducting periods of the primary switch. Improvements in power factor and input waveform are accomplished as the inductor current has an amplitude in proportion with that of the a.c. input voltage.
The conduction of the ancillary switch, on the other hand, will result in current flow through the additional transformer winding, which is a tertiary in the preferred embodiments. Then a sufficient voltage will develop across the transformer primary to cause the soft-switching capacitance means to discharge, with a consequent drop in the voltage across the primary switch. A zero-voltage turn-on of the primary switch is thus realized for reduction of switching loss and noise.
The primary switch operates both for improvements in power factor and input waveform and for d.c.-to-d.c. conversion. The objectives of improved power factor and improved input waveform in view are thus attained with little or no addition to the size or manufacturing cost of the switching power supply.
It will also be appreciated that the winding included in the soft switching circuit is incorporated with the transformer as a tertiary in the preferred embodiments. This feature also contributes to the compactness of the device.
A further feature of the invention resides in an ancillary charging circuit connected between a third output of the rectifier circuit and the smoothing capacitor. The third output of the rectifier circuit puts out substantially the same rectifier output voltage between itself and the noted second output of the rectifier circuit as that between the first and the second output thereof. The ancillary charging circuit comprises another ancillary winding (quaternary in the preferred embodiments) of the transformer. Various specific designs will be proposed for the ancillary charging circuit.
The ancillary charging circuit is well calculated to charge the smoothing capacitor to the required degree even if the current through the primary inductor, which is for improvements in power factor and input waveform, is lessened in magnitude. The smoothing capacitor is charged both via the primary inductor and via the ancillary charging circuit. As a result, the current charging the smoothing capacitor via the primary inductor can be made less by an amount equal to the current charging the smoothing capacitor via the ancillary charging circuit than if the smoothing capacitor were charged via the primary inductor only. Power loss at the primary inductor is thus decreased, and its size can be reduced.
The ancillary charging circuit may also be utilized to make higher the voltage under which the smoothing capacitor is charged. This will serve to prevent the flow of overcurrent into the smoothing capacitor via the primary inductor at or adjacent the peaks of the a.c. input voltage. The result will be the reduction of higher harmonics of the a.c. input voltage.
An additional advantage of the ancillary charging circuit is that it makes use of an ancillary winding of the transformer. The ancillary charging circuit can therefore be most simplified in construction and reduced in size.
The transformer primary is tapped in some preferred embodiments of the invention, and the primary inductor is connected between the first output of the rectifier circuit and the smoothing capacitor via part of the transformer primary. Current will then flow through the primary inductor only when its input voltage is higher than the tap voltage, resulting in less power loss at the primary inductor.
In other embodiments, however, the transformer primary is not tapped, and the primary inductor is connected between the rectifier circuit first output and the smoothing capacitor via whole of the transformer primary. This connection is recommended where higher power factor and better input waveform are more important than efficiency, cause then the current through the primary inductor is not limited by the transformer primary.
The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference had to the attached drawings showing the preferred embodiments of the invention.